The present invention relates generally to the fabrication of magnetoresistive sensors or transducers for data storage applications, and more particularly to a non-destructive analysis of magnetoresistive sensor dimensions as viewed from an air bearing surface to ensure that a desired magnetoresistive sensor height has been achieved.
Conventional lap monitors for inductive thin film heads are typically used to lap a bar of inductive heads to a final height. These conventional monitors typically utilize pole metalizations and polymers to form an analog-digital system for end point lap detection. The metalizations are common to the thin film inductive head write structures. However, with sliders having a magnetoresistive (MR) sensor design positioned on the surface of the slider to read information from a disc, the situation is much more complicated since the head is a combined MR read, inductive thin film write device. For proper MR reading, it is essential to control the end point lap detection of the slider surface bearing the MR sensor by using features common to the MR sensor. The end point lap detection determines the final height of both the slider rail and the MR sensor. Since inductive thin film lap monitors utilize resistors having features common to inductive thin film writers, end point lap detection control of the slider and its MR sensor is degraded if writer base lap monitor resistors are used.
Sliders having MR sensors are used in magnetic storage systems to detect magnetically encoded information from a storage medium, i.e. to read information from a disc. A time dependent magnetic field from the magnetic storage medium or disc directly modulates the resistivity of the MR sensor. In particular, changing magnetic fields originating from the magnetic storage medium rotate the magnetization of the MR sensor and thereby change the resistance of the sensor. This phenomenon is called the MR effect. The change in resistance of the MR sensor can be detected either by passing a sense current through the MR sensor and measuring the voltage across the MR sensor, or by passing a voltage across the MR sensor and measuring the current through the MR sensor. The resulting signal can be used to recover information from the magnetic storage medium.
During the fabrication of magnetic heads for use in magnetic data storage applications, an array of sensors and auxiliary circuits are fabricated on a common substrate in a deposition of metallic and non-metallic layers. In most fabrications, there is an auxiliary circuit for each sensor. Patterning the array of sensors and auxiliary circuits is accomplished using photolithography in combination with etching and lift-off processes. The finished array or wafer is then optically and electrically inspected and subsequently cut into small arrays, rows or bars. Next, individual bars of sensors and auxiliary circuit are machined at a surface which will eventually become the air bearing surface of the sensor until the auxiliary circuit indicates that a desired MR sensor height has been obtained.
During machining of a particular row of sensors and auxiliary circuits, the air bearing surface moves from a beginning position to a final position, while reducing the height of the sensors. The primary function of the auxiliary circuits is to stop the machining process once the auxiliary circuit indicates that the desired sensor height has been achieved. After a particular row of sensors is machined, the row can be cut or diced into individual sliders. During this process, the auxiliary circuits can be destroyed if desired. U.S. Pat. No. 5,463,805 entitled "Method of Lapping MR Sensors" discloses one type of an auxiliary circuit for estimating the proper sensor height. This patent discloses an auxiliary circuit which utilizes a reference resistor, a target resistor and a variable resistor. The resistors are formed from MR material.
There are numerous difficulties associated with conventional MR auxiliary circuit designs. One common problem with these circuits is that they do not precisely account for errors introduced by the existence of mask or contact edge movement during wafer processing steps. The edge movement phenomena caused by wafer processing results in the magnetoresistive resistors of the auxiliary circuit being reduced or expanded in size as surfaces or edges of these elements move by an undetermined quantity. Thus, the actual lengths and heights of these elements will frequently be different then the intended length and heights (i.e., the mask lengths and heights). The dimension changes in the sensor and in the resistors of the auxiliary circuit introduce errors in the machining process. For example, if the resistance of a reference resistor in an auxiliary circuit is dimensionally changed by edge movement, comparison of its resistance to the resistance of a target or variable resistor of the sensor will result in the machining process being halted at the wrong time which would produce a row of sensors having an undesired height.
A second common problem associated with conventional MR auxiliary circuit designs is due to the lapping of an entire bar of sensors at one time. If the row of sensors is not coplanar with the lapping or machining device, each particular sensor of the row of sensors will be machined or lapped to a different sensor height. Thus, the entire row of sensors could have an improper height, and thus would not correspond to necessary tolerances.
Thus, there is a need for an additional system which can be utilized to verify whether a proper sensor height has been obtained once the MR sensor and auxiliary circuit have been lapped. However, it is important that the measurement is preformed without destroying the sensor.